Gluten is a complex mixture of proteins from wheat. Traditionally, it has held a unique place as a protein additive in yeast-raised baked goods due to its ability to form extensible tough films and maintain this structure during the baking process. It has also been used in other food processing areas such as in the manufacture of pasta, breakfast cereals, vegetarian diet foods, portion control foods such as pies and in the manufacture of smallgoods.
Wheat gluten holds approximately twice its own weight in water when fully hydrated under conditions where considerable energy is imparted as, for example, in high speed mixing. However, when dry gluten is hydrated with minimal work, it adheres strongly to itself to form a ball with approximately its own weight of water. Gluten exhibits poor emulsifying activity, i.e. it does not promote the formation of stable emulsions nor does it produce stable foams. It is largely insoluble in water at neutral pH but shows good solubility at acidic pH.
The uses of gluten (wet or dry) have been restricted therefore in those applications which require good emulsifying properties, high water holding capacity, the formation of stable foams or high solubility.
Various methods have been investigated and proposed to alter the functional characteristics of wheat gluten. Desired properties resulting from such modifications include improved solubility, improved ability to emulsify fats, improved ability to whip and other properties apparent to those skilled in the art.
Thus one known process involves treatment of wheat gluten with alkaline chemicals at varying temperatures and times. Such methods claim to remove primary amides from glutamine and asparagine residues and thus increase the solubility of wheat gluten. These processes may or may not include a neutralisation step after reaction. However, while the process is simple to execute, it involves the use of highly corrosive chemicals and in some cases evolution of toxic and corrosive gases such as H.sub.2 S and NH.sub.3. The products frequently contain short chain peptides which by their chemical nature and sequences are known to impart bitter flavours to the modified gluten Other detrimental changes include unpleasant odours and colours in the final products. [Batey and Gras (1981) J Fd-Technol 16 561-566 ]
Another known process includes the use of acids to effect deamidation of the side chains of the gluten. These processes may or may not involve a neutralisation step prior to drying. Such processes are claimed to result in improved solubiltity whipping properties and emulsification, but frequently involve hydrolysis and denaturation of the gluten proteins during or prior to the modification. Denaturation results in disruption to the primary, secondary, tertiary and quaternary structures of the proteins. Such processes may result in products which like those from alkaline modifications have bitter flavours, and unpleasant odours. Similarly, while such processes have utilised pH and pI properties of the wheat proteins to effect modifications, they are frequently unecomonic to perform commercially. [Wu et al (1976) J. Agric Food Chem 24: 504; Finley, J. W. 1975 J. Fd Sci 40 1283; Yagi et al (1985) European Patent 0 164 929
Another group of known processes includes the use of enzymes notably proteases in alkaline, neutral or acid conditions. Proteases used have included those from vegetable, animal or microbial sources. Enzymically modified glutens may result from controlled hydrolysis reactions in which the enzyme may be finally inactivated prior to drying or from non regulated hydrolysis reactions where enzyme inactivation does not occur and residual enzyme remains in the final product. Enzyme inactivation is generally achieved by pH or temperature adjustment. Increased solubility occurs in most enzymically modified glutens, but there is often a major reduction in molecular size of the hydrolysed protein. Prior art indicates that in some enzyme modifications denaturation of the gluten proteins occurs as part of the reaction and the products frequently contain bitter flavours. [Batey J (1985) Appl. Biochem 7 423-429; Verma & McCalla (1966) Cereal Chem 43 28; Novo Industri Process Development Bulletin #6 February 1988 ]
Finally other known processes for the modification of gluten include the preparation of salts of gluten including succinates, phosphates, sulfates.
Gluten salts are generally produced under extremes of pH and temperature both of which would be expected to produce changes in gluten structure. [Knight J. W. 1965 The Chemistry Of Wheat Starch and Gluten p92-94 ]
Succinylation has been achieved using succinic acid in 1, 4 dioxane at low temperatures. [D. R. Grant 1973 Cereal Chem 50 417 ]
All known gluten modifications have resulted in alterations of the structure of the gluten which may affect the primary and secondary structure of the protein as well as the tertiary and quaternary. Denaturation of the protein occurs prior to hydrolysis or modification occurring.
Although some improvements in functional characteristics have been noted, the production methods have often been uneconomic, the products frequently possess "off-flavours" and odours which would be unacceptable. Combinations of modified glutens with proteins from other animal and vegetable sources have been proposed. (Batey IL & Gras PW, (1983) Food then 12, 265; Yagi, N et al (1985) European Patent 0 164 929).
It has been surprisingly discovered by us that when gluten is subjected to much less severe conditions for relatively short periods of time at slightly elevated temperatures, but below the known temperature of denaturation, the resulting product exhibits marked improvements in functional properties such as water holding capacity, emulsification and foaming properties.
Little attention has been paid to the effect of mild conditions on changes to the functionality of gluten, nor to the residual gluten after modification to increase solubility. It has been surprisingly discovered, that when gluten is subjected to acid conditions for short periods of time at slightly increased temperatures, the functional properties of the resulting product are markedly improved in terms of water holding capacity, emulsification and foaming.